Humans tend to make changes when there are no other options... and often, after significant damage has been done. Presumably if we "stop producing enough food to sustain us", a lot of humans would die off, but some would survive on the reduced food supply, and make changes to prevent further loss of food. Bear in mind, it's incredibly unlikely that all possible human food sources die off simultaneously.

A good example of change in behavior to ensure survival, is that major powers used to go much more openly to war for territory and resources right up until those powers got the ability to utterly destroy humanity. While there are still countries who are enemies of each other, and who fight small proxy wars and the like, nobody in their right mind would stage an invasion of another major power, likely ever again.